The use of reflected light received by optical fibers to measure displacement of a target is well known. Known probes generally comprise a bundle of optical fibers some of which transmit light to a target. A portion of the light which strikes the target is reflected back to the probe. The reflected light is received by and communicated along other optical fibers in the probe to a transducer which then generates an electrical signal proportional to the light received. As the target distance increases, the intensity of reflected light decreases. Thus, changes in the signal output of the light sensor can serve as a direct measure of changes in target distance. Typical examples of such sensors are disclosed in U.S. Pat. Nos. 3,327,584; 3,940,608; 4,247,764; 4,694,160; and 4,701,611. Unfortunately, measurement of displacement based upon changes in absolute light intensity greatly impairs the usefulness and sensitivity of such known probes. First, different targets will have different types of surfaces with different reflectivities, thereby necessitating frequent recalibration. Second, the light source itself may not always remain constant, but rather may often displays short-term variance resulting from variation the power supply. In addition, light sources frequently exhibit a decrease in intensity over time. Such changes require either frequent recalibration or the addition of costly circuitry and/or light sources to insure that the amount of light initially transmitted remains nearly constant. Finally, crosstalk between the light emitting and light receiving fibers in mixed bundles considerably reduces the resolving power of the apparatus. Although many modifications have been proposed to overcome these problems, there has been little improvement.
More recently, U.S. Pat. No. 4,701,610 described a fiber optic probe in which some of the light receiving fibers are segregated from a mixed bundle of light emitting and light receiving fibers. This provides two measures of reflectivity, one based on the segregated receiving fibers and one based on the receiving fibers which remain intermixed with the light emitting fibers. These two signals can then be processed to provide an output which is somewhat independent of changes in target reflectivity. Although this probe suffers from fewer of the problems encountered with reflectivity dependent probes, many difficulties remain. Crosstalk between the light emitting and light receiving fibers of the mixed bundle is particularly problematic, considerably reducing the probe's capacity for resolution. Furthermore, having a bundle of mixed light emitting and light receiving fibers substantially decreases the linearity of the final output signal, thereby introducing error and decreasing the range of distance through which the probe can effect an accurate measurement. Thus, there exists a need for a fiber optic probe which can determine the displacement of a target accurately over very small changes is distance, and which will provide a measurement which is substantially independent of target reflectivity and source illumination variation.